Electrophotographic charge transport layer

ABSTRACT

AN ELECTROPHOTOGRAPHIC PLATE IN WHICH THE PHOTOCONDUCTOR IS OVERCOATED WITH A CHARGE TRANSPORT LAYER OF POLYAMIDE WHICH IS FORMED FROM LINOLEIC ACID AND ETHYLENE DIAMINE AND WHICH HAS A RESISTIVITY OF AT LEAST 10**10 OHM CM. AND A DARK DECAY TIME OF LESS THAN TEN SECONDS.

United States Patent 3,713,820 ELECTROPHOTOGRAPHIC CHARGE TRANSPORT LAYER Robert B. Champ, San Jose, Albert J. Cherry, Morgan 1 1111, and Meredith D. Shattuck, San Jose, Calif., assrgnors to International Business Machines Corporation, Armonk, N.Y.

No Drawing. Continuation-impart of abandoned applicatron Ser. No. 886,786, Dec. 19, 1969. This application Sept. 7, 1971, Ser. No. 178,493

Int. Cl. G03g 5/02, 13/22 US. C]. 96-15 6 Claims ABSTRACT OF THE DISCLOSURE An electrophotographic plate in which the photoconductor is overcoated with a charge transport layer of polyamide which is formed from linoleic acid and ethylene diamine and which has a resistivity of at least 10 ohm cm. and a dark decay time of less than ten seconds.

This application is a continuation-in-part of copendrng application Ser. No. 886,786, filed Dec. 19, 1969 and now abandoned.

FIELD OF THE INVENTION The present invention is concerned with electrophotographic plates and their use. In particular, it is concerned with providing and using a novel type of overcoating for an electrophotographic plate. The overcoating comprises polyamide of the specified type having a specified resistivity and a specified dark decay time. It has an unexpected advantage over prior art overcoatings in that it acts as a charge transport layer, thereby avoiding the buildup of residual charge so that the electrophotographic plate may be used repeatedly without the necessity of a charge reversal step inbetween each use.

PRIOR ART SUMMARY OF THE INVENTION It has now been discovered that those polyamides which are formed from linoleic acid and ethylene diamine and which have a resistivity above 10 ohm cm. and a dark decay time of less than 10 seconds are uniquely suitable for use as charge transport overcoating for an electrophotographic plate. Many polyamides do not have such resistivities, but it is essential that the resistivity be above 10 ohm cm. When the resistivity is too low, the material is too much of a conductor and suffers the disadvantage of lateral conduction. For optimum results, the resistivity is preferably approximately 10 ohm cm.

It is also essential that the polyamide have a dark decay time of less than ten seconds. When the dark decay time is more than 10 seconds, the polyamide acts as an insulator, and is not suitable for use in the present invention.

Polyamides formed from linoleic acid and ethylene diamine are commercially available from a number of sources. The polyamides are usually prepared from dimers or trimers of the acid, by thermal polymerization with the ethylene diamine.

Surprisingly, it is not sufficient merely to use any organic polymeric material with the specified resistivity. Our work leads us to the belief that the specified polyamides are unique in their ability to inject the corona surface charge and to act as a charge transport layer. Even other organic polymers having the specified resistivity do not act as charge transport layers. The explanation for this is not fully understood and the result was very unexpected.

The charge transport layer of the present invention may be used with any of the photoconductive layers useful in electrophotography. For example, the photoconductive layer may suitably be of selenium or selenium and tellurium mixtures, anthracene, or other organic photoconductors. It may also be of inorganic materials, such as Zinc oxide, cadmium selenide, cadmium sulfide, or other phosphors with or without binder material, such as organic polymeric resins.

No particular method of applying the charge transport layer is required by the present invention. The preferred method is simply applying the polyamide from a solvent. The layer, however, may suitably be applied by any other known method, for example, by ion bombardment under partial vacuum, by lamination or by application from a hot melt. In general, a thickness of about 7 microns is preferred for the charge transport layer, although it may be as thin as about 2 microns and, most surprisingly, it may also be as thick as about 50 microns. Prior to the present invention, no overcoatings of such thickness were possible.

The measurement of resistivity is a routine procedure well known in the art. It should be mentioned, however, that the resistivity may vary depending upon the temperature and the humidity at which the measurement is made. It is therefore advisable to conduct the measurement of the resistivity under the same conditions as those to be encountered when the plate is in use. In measuring the resistivity, some care must be exercised to insure that the observed conduction is a bulk property of the material and is not due to carriers injected from the electrodes. For example, the following procedure may be used. A polyamide film of approximately 10 microns thickness is coated on a conducting substrate and mounted between two spring-loaded probes of approximately 1 cm. surface area. The surfaces of the probes are coated with a thin film of mercury to insure a uniform electrical contact to both the surface of the polyamide film and the conducting substrate backing it. The current, which passes through this package as a function of the voltage applied across it, is then determined using a sensitive ammeter. When a polarity-independent linear current-voltage relation is observed, the slope of this line is taken to be the resistance of the polyamide film. The bulk resistivity is then calculated.

Dark decay time is a standard measurement well known in the art. It is the time required for a Sample in the dark in contact with a conducting substrate to lose onehalf of the original voltage applied by corona charging. To determine the dark decay time for a polyamide film, the following procedure may be used. The polyamide is coated on a conducting substrate and placed in a rotating-disk electrometer which alternately subjects the film to corona charging and measures the potential across it. When the potential becomes stable, the corona charging step is omitted and the decay of the potential across the film is observed. Failure of the film to develop any potential under corona charging is interpreted as being done to a very short decay time.

We have found that certain polyamides ordinarily having dark decay times longer than seconds may have their dark decay times lowered to Within the required range by adding a small'amount of certain activators, such as electron acceptor materials, like Alizarin Yellow GG, p-chloranil, tetracyanoethylene, and tetrabromophthalic anhydride. It is postulated that these sensitizing materials operate by forming charge transfer complexes with the polyamides, thereby lowering the dark decay time. In general, from trace amounts up to about 1% or so is sufiicient. Other sensitizers useful in the present invention include those dyes listed in US. Pats. 3,037,861, 3,169,060 and 3,287,113.

The advantages of overcoating have been appreciated by the art for some time. These include an increased ability to resist wear, humidity, and filming, and superior surface properties due to greater smoothness, particularly when the photoconductor layer is non-homogeneous. Despite these recognized advantages, however, the currently used commercial electrophotography methods do not employ overcoating, because the previously known overcoatings suffered from the disadvantage of residual charge. The present invention, however, provides a method of obtaining all the advantages of an overcoat Withoutthis disadvantage.

PREFERRED EMBODIMENTS The general nature of the invention having been set forth, the following examples are now presented as to the specific preparation of preferred embodiments of the invention. The specific details presented are for purposes of illustration and not limitation.

EXAMPLE I G. CdS powder 6 46950 adhesive 3 (46950 adhesive is a polyethylene terephthalate adhesive from Du Pont.)

The thickness of the above sample is approximately 50 The total plate is cured at 100 C. for 10 minutes and is then laminated at 290 F., 3 feet per minute and 50 p.s.i.g. to an aluminum (6 mil) rough stock coated with 2n 46950 adhesive. The Mylar is then stripped away leaving the polyamide transport layer over the CdS/binder photoconductive layer.

Excellent electrophotographic characteristics are obtained from said plate (i.e., 800 v. dark charge; speed 3 times a commercial selenium sample and good copy quality). Slight residual potentials are noted in this system but addition of electron acceptor compounds, such as Alizarin Yellow GG, p-chloranil or tetrabromophthalic anhydride, decrease the residual to a negligible amount. When Emerez 3794R isused, no residual is noted, even without addition of sensitizers (Emerez 3794R, a thermoplastic polyamide resin from Emery Industries, Inc., is prepared from linoleic acid and ethylene diamine by thermal polymerization).

For purposes of comparison, a 6 overcoat of polyethylene terephthalate was made as described above. Residual potential was initially high and increased on cycling, showing this polymer not to be a transport layer.

4 Cycle: Percent residual 1st 29 5th EXAMPLE II Results similar to those shown in Example I above were obtained when the photoconductive layer was made from 1.5 g. CdS in 1.0 g. of 10% polyvinylcarbazole in tetrahydrofuran.

EXAMPLE III Polyamide over an organic photoconductor Poly N-vinylcarbazole in tetrahydrofuran is mixed in a 1:1 molar ratio with 2,4,7 trinitro-9-fiuoronone. The resultant solutionis coated 101.4. thick on an aluminum plate and allowed to cure at 55 C., for 4 hours. A 5,u coating of Emerez 2794R polyamide, 20% in toluene, was coated on Mylar and subsequently after curing for 5 minutes at C. was laminated, the polyamide in contact to the photoconductor surface, with a hot roll at 250 F., 30 p.s.i., and 3 feet per minute to the organic photoconductor surface. The Mylar was then stripped away leaving the photoconductive element comprising the polyamide transport layer and the organic photoconductor. The following electrophotographic results were obtained, and they are shown compared to a control plate which differs only in having no polyamide overcoat.

EXPOSING SOURCE: TUNGSTEN LAMP NorE.- C.A. stands for Charge Acceptance; T%=half-life, i.e., the time required to discharge in light to one-half of original potential.

EXAMPLE IV Polyamide over selenium A 7 transport layer comprising Versalon 1175 polyamide resin with 1% p-choranil added as a sensitizer was coated from 87% CHCl 13% CH OH on 5 mil Mylar. This sample was cured for 5 minutes at 100 C. and then was laminated to a commercially available selenium plate, the polyamide layer in contact with the selenium layer. Laminating conditions were 250 E, 40 p.s.i., and 3 feet per minute. After cooling the Mylar was removed leaving the photoconductive element comprising the commercial selenium plate with a 7,u transport overlayer of polyamide resin. Electrophotographic data show such a plate to have no increase in residual potential and no ill efifects on other electrophotographic parameters (i.e. charge acceptance, dark decay, etc.).

For the purpose of comparison, a plate was made as above using polyethylene terephthalate as a 6,111. overcoat over the commercial selenium plate. A 37% residual potential was observed on the initial electrophotographic cycle and this increased to 85% on the 5th cycle rendering the plate unusable in the electrophotographic mode.

EXAMPLE V Difierent polymeric overcoats were made for purposes of comparison.

Plate fabrication Polymers are coated from a 10% solids solution in tetrahyrofuran on one mil Mylar. Thickness is 8 CdS/46950, 10 to 1 pigment to binder ratio is coated on Al.

Lamination of polymer to CdS is done at 250 F., 30 p.s.i.g., 3 feet per minute and Mylar is stripped.

Polymer: Percent residual potential Polyvinylidine chloride 50 Cellulose nitrate 42 Methyl methacrylate-acrylic acid copolymer 38 Polyvinyl butyral 88 Vinyl chloride-vinyl acetate copolymer 94 Polyvinyl acetate 82 None of these polymeric overcoats is usefiul, because of the high residual potential.

What is claimed is:

1. An electrophotographic plate comprising a photoconductor layer overcoated with a charge transport layer from about 2 to about 50 microns thick of polyamide which is formed from linoleic acid and ethylene diamine and which has a resistivity above about ohm cm. and a dark decay time of less than 10 seconds.

2. A plate as claimed in claim 1 wherein the resistivity of the polyamide is approximately 10 ohm cm.

3. A plate as claimed in claim 1 wherein the photoconductive layer comprises an organic photoconductor.

4. A plate as claimed in claim 1 wherein the photoconductive layer comprises an inorganic photoconductor.

5. A plate as claimed in claim 1 wherein a Lewis acid sensitizer has been added to the polyamide.

6. A process for [forming an electrostatic charge pattern comprising charging and selectively illuminating in accordance'with an image pattern an electrophotographic plate comprising a photoconductor layer overcoated with a charge transport layer from about 2 to about microns thick of polyamide which is formed from linoleic acid and ethylene diamine and which has a resistivity above about 10 ohm cm. and a dark decay time of less than 10 seconds.

References Cited UNITED STATES PATENTS 3,598,582 10/ 1971 Herrick et a1 961.5 3,634,079 1/ 1972 Champ et a1 961.5

CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R. 

